Human growth hormone, Somatotropin, a substance produced in the pituitary gland and needed for growth throughout childhood and adolescence. In this image, produced from x-ray crystallographic data, GH (orange ribbons in foreground) is shown bound to its receptor (dark brown ribbons in background). A disulfide bond, formed by two cysteine amino acids (spheres) is shown in foreground. Disulfide bonds crosslink and stabilize the internal structure of proteins.